743 research outputs found
NGC 3105: A Young Cluster in the Outer Galaxy
Images and spectra of the open cluster NGC 3105 have been obtained with GMOS
on Gemini South. The (i', g'-i') color-magnitude diagram (CMD) constructed from
these data extends from the brightest cluster members to g'~23. This is 4 - 5
mag fainter than previous CMDs at visible wavelengths and samples cluster
members with sub-solar masses. Assuming a half-solar metallicity, comparisons
with isochrones yield a distance of 6.6+/-0.3 kpc. An age of at least 32 Myr is
found based on the photometric properties of the brightest stars, coupled with
the apparent absence of pre-main sequence stars in the lower regions of the
CMD. The luminosity function of stars between 50 and 70 arcsec from the cluster
center is consistent with a Chabrier lognormal mass function. However, at radii
smaller than 50 arcsec there is a higher specific frequency of the most massive
main sequence stars than at larger radii. Photometry obtained from archival
SPITZER images reveals that some of the brightest stars near NGC 3105 have
excess infrared emission, presumably from warm dust envelopes. Halpha emission
is detected in a few early-type stars in and around the cluster, building upon
previous spectroscopic observations that found Be stars near NGC 3105. The
equivalent width of the NaD lines in the spectra of early type stars is
consistent with the reddening found from comparisons with isochrones. Stars
with i'~18.5 that fall near the cluster main sequence have a spectral-type A5V,
and a distance modulus that is consistent with that obtained by comparing
isochrones with the CMD is found assuming solar neighborhood intrinsic
brightnesses for these stars.Comment: To appear in The Astrophysical Journa
The Open Cluster NGC 2437 (Messier 46)
The stellar content of the open cluster NGC 2437 (Messier 46) is investigated
using moderately deep u*,g', and r' MegaCam images. When compared with solar
metallicity isochrones, the (g', u'-g') and (r', g'-r') CMDs are consistent
with an age log(t_yr) = 8.35 +/- 0.15, a distance modulus 11.05 +/- 0.05, and a
color excess E(B-V) = 0.115 +/- 0.035. The r' luminosity function (LF) of main
sequence stars in the magnitude range r' 0.8 solar) has a
shape that follows solar neighborhood star counts. However, at fainter
magnitudes the cluster LF is flat, in contrast with what would be expected from
solar neighborhood counts. The clustering properties of stars in NGC 2437 are
investigated by examining the two-point angular correlation functions of main
sequence stars in different brightness ranges. Main sequence stars fainter than
r' = 17 are less centrally concentrated than brighter stars and are found over
a larger area of the sky, suggesting that there is a corona of faint main
sequence stars around NGC 2437. Based on the flat LF and extended spatial
distribution of faint stars, it is concluded that NGC 2437 is actively shedding
stars with masses < 0.8 solar.Comment: To appear in the PAS
Haffner 16 Redux: Revisiting a Young Cluster in the Outer Galaxy
Images and spectra recorded with the Gemini Multi-Object Spectrograph on
Gemini South are used to investigate the stellar content of the open cluster
Haffner 16. The (i', g'-i') color-magnitude diagram (CMD) constructed from
these data samples the cluster main sequence (MS) and 5 magnitudes of the
pre-MS (PMS). The isochrones do not track the PMS on the CMD, in the sense that
the PMS has a shallower slope on the CMD than predicted by the models. Still, a
dip in star counts that is associated with the relaxation of PMS stars onto the
MS is identified near i' = 17. The depth and brightness of this feature - as
well as the morphology of the cluster MS on the CMD - are matched by models
with a slightly sub-solar metallicity that have an age ~ 20 Myr and a distance
modulus of 12.3 +/- 0.2. A light profile of Haffner 16 is constructed in the
WISE W1 filter which suggests that the cluster is surrounded by a diffuse
stellar halo. Spectra are presented of candidate cluster MS and PMS stars
selected according to location on the CMD. The spectra show characteristics
that are suggestive of a sub-solar metallicity. Halpha emission is common among
objects on the PMS locus on the CMD near i' = 18. It is suggested that the
location of the Haffner 16 PMS on the CMD is affected by large-scale cool spot
activity, likely induced by rapid stellar rotation.Comment: 30 pages total, with 23 figures. The format differs slightly from the
published articl
The Distribution of Main Sequence and Pre-Main Sequence Stars in the Young Anticenter Cluster NGC 2401
Images obtained with the Gemini Multi-Object Spectrograph on Gemini South are
used to examine the photometric properties and spatial distributions of main
sequence (MS) and pre-main sequence (PMS) objects in the star cluster NGC 2401.
The data sample several magnitudes fainter than previous studies, and a large
population of candidate PMS (cPMS) stars are identified. The cPMS stars are
traced out to 2.4 arcmin from the cluster center, and have a flatter spatial
distribution than the brightest MS stars near the cluster center. The
luminosity function of all MS and candidate PMS stars can be matched by a model
that assumes a solar neighborhood mass function, suggesting that NGC 2401 has
not yet shed significant numbers of members with masses in excess of 0.5 solar.
The frequency of wide binaries among the MS stars is ~3 times higher than among
the cPMS stars. It is argued that the difference in the spatial distributions
of MS and PMS objects is not the consequence of secular dynamical evolution or
structural evolution driven by near-catastrophic mass loss. Rather, it is
suggested that the different spatial distributions of these objects is the
fossil imprint of primordial sub-clustering that arises naturally if massive
stars form preferentially in the highest density central regions of a
protocluster.Comment: To appear in the PAS
NGC 5253 and ESO 269-G058: Dwarf Galaxies with a Past
Deep r' and i' images obtained with GMOS on Gemini South are used to probe
the bright stellar content in the outer regions of NGC 5253 and ESO269-G058.
Red giant branch (RGB) stars are traced out to a distance of 8 kpc along the
major axis of NGC 5253, and 6 kpc in ESO269-G058. The outer regions of both
galaxies are metal-poor; RGB stars located between projected major axis
distances of 2 and 4 kpc in NGC 5253 have [Fe/H] ~ -1, whereas RGB stars in the
corresponding portion of ESO269-G058 have [Fe/H] ~ -1.8. Stars with
metallicities that differ from the mean by more than a few tenths of a dex make
only a modest contribution to the stellar content in the outer regions of both
galaxies. A population of bright asymptotic giant branch (AGB) stars is seen in
both galaxies. Roughly 1 - 10% of the stellar mass of NGC 5253 may have formed
during the past few hundred million years, and it is suggested that the
progenitors of the two recent SN Ia in this galaxy may have formed at this
time. It is argued that the current episodes of star formation in NGC 5253 and
ESO269-G058 may have been triggered up to ~ 1 Gyr in the past. Finally, a
distance modulus is computed for each galaxy based on the brightness of the
RGB-tip, and the results are 27.48 +/- 0.14 for NGC 5253, and 27.93 +/- 0.18
for ESO269-G058.Comment: 36 pages, including 11 postscript pages. Astronomical Journal in
pres
Ghosts in the Attic: Mapping the Stellar Content of the S0 Galaxy NGC 5102
The spatial distribution of stars in the nearby S0 galaxy NGC 5102 is
investigated using images obtained with WIRCam and MegaCam on the
Canada-France-Hawaii Telescope. With the exception of gaps between detector
elements, the entire galaxy is surveyed in r' and i', while the J and Ks data
extend out to 6 kpc (7 disk scale lengths). A modest population of main
sequence (MS) stars with M_V < -3.5 and ages 70 Myr are detected throughout the
disk, with the majority located in the southern half of the galaxy. The ratio
of C stars to bright M giants is consistent with an overall increase in the
star formation rate within the past 1 Gyr. Star-forming activity during the
interval 0.1 - 2 Gyr was more centrally concentrated than during the past 100
Myr. The structure of the disk changes near 5 kpc (5.5 disk scale lengths).
RSGs and bright AGB stars are traced out to a radius of 14 kpc (15.6 scale
lengths) along the southern portion of the major axis, while a tentative
detection is also made of bright AGB stars at a projected distance of 16 kpc
along the south east minor axis. A large clump of AGB stars that subtends an
arcmin is identified to the west of the galaxy center. It is argued that this
is the remnant of a companion galaxy that triggered past episodes of elevated
star-forming activity.Comment: Accepted for publication in the A
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